A V shaped loading operation, which is a typical excavating pattern of a working machine, for example, a wheel loader, will be explained with FIGS. 7 and 8. An excavating process, occupying about 4 seconds out of about 25 seconds per cycle, determines the excavating performance as to how quickly and strongly a bucket edge, which is an example of a working machine, is penetrated into earth and sand in about 1 second in which the operation "from penetrating into the ground to separating from the ground surface" is conducted. Accordingly, an optimization of the force balance of a bucket edge and an improvement in speed (responsiveness) of about 1 second are important.
A first prior art (which corresponds to FIG. 7 in Japanese Laid-open Patent No. 9-32045) will be explained with reference to FIG. 9. The oil pressure, supplied to the working machine cylinder 9 from the working machine pump 6 via the working machine hydraulic circuit 8, increases only to the set pressure of the working machine relief valve 38 during an excavating operation. Therefore, there are situations in which the force for lifting a bucket is in short supply such that the bucket can not be lifted. In such cases, the oil pressure of the working machine cylinder 9 operates on an unloading valve 66 to unload the working machine hydraulic circuit 8, and at the same time a pressure sensor 69 detects that the pressure of the working machine hydraulic circuit 8 is not less than a predetermined pressure, whereby an operator moves a switch 68 to its ON position. Thus, when a signal from the pressure sensor 69 and an ON signal from the switch 68 are inputted to the AND circuit 67, the solenoid 65a of the on-off valve 65 is electrified so that the on-off valve 65 is switched to its communicating position. Hence, the oil pressure of the working machine cylinder 9 operates on the working machine assisting valve 64 via the conduit 71 to switch the working machine assisting valve 64 to its communicating position. Consequently, the high discharge pressure of the travel pump 2 is supplied to the working machine cylinder 9 by way of the check valve 72, the working machine assisting valve 64, and the conduit 71, whereby the bucket can be lifted by the increased thrust.
In a second prior art for increasing the working machine force, the working machine pump is changed from a gear pump to a plunger pump and the oil pressure is raised, for examples from 210 kg/cm.sup.2 to 320 kg/cm.sup.2, thus increasing the working machine force.
In a third prior art for increasing the driving force of the vehicle, the loss in oil pressure is reduced by unloading the whole or a part of an unnecessary pumped quantity at the time of operation with a multistage pump, for example, or the loss in oil pressure is reduced by reducing the discharged quantity from a variable displacement pump. Thus, the driving force of the vehicle is increased by the reduced oil pressure loss, and the excavating performance is improved.
However, the aforesaid prior arts have the following disadvantages.
(1) In the first prior art, when a signal from the pressure sensor 69 and an ON signal from the switch 68 are inputted to the AND circuit 67, the solenoid 65a of the on-off valve 65 is electrified so that the on-off valve 65 is switched to its communicating position, and the working machine assisting valve 64 is switched to its communicating position. However, considerable time is needed before the working machine assisting valve 64 is switched to its communication position, after the signal from the pressure sensor 69 and the ON signal from the switch 68 are inputted to the AND circuit 67. Therefore, there is a disadvantage in that it is difficult to attain both the optimization of the force balance of the bucket edge and the improvement in speed (responsiveness) in about 1 second, thereby lowering the excavating performance.
(2) Moreover in the first prior art, while the working machine assisting valve 64 is switched to its communicating position, the oil pressure of the working machine cylinder 9 operates on the unloading valve 66 to unload the working machine hydraulic circuit 8. Therefore, there is a merit in that the whole engine torque turns into driving torque of the travel pump 2, thus increasing the tractive force of the vehicle. However, there is a disadvantage in that, in slippery working sites and the like, tire slips occur, thus increasing the abrasion of the tires and lowering the excavating operational efficiency.
(3) In the second prior art, the pressure is always raised so that reinforcement of power lines, such as an accelerator and the like, is needed. In addition, the plunger pump has a higher cost as compared with the gear pump. When the plunger pump, for example, is used, there is a disadvantage in that the merits, produced by using the more expensive plunger pump, are not utilized if the pressure is usually at a low pressure value (e.g., 210 kg/cm.sup.2) and turns into a high pressure value (e.g., 320 kg/cm.sup.2) when necessary, but not frequently with variable relief and the like.
(4) The multistage pump and the variable displacement pump used in the third prior art increase the cost.